United States patent application US 2009/0210096 discloses a data centre comprising a computer room air conditioner and an air economiser. An air economiser or outside air economiser is a system that cools a building using air from outside the building. This system is most effective when the outside air is cooler than the air inside. The air exhausted by the air economiser flows along chilled water valves and compressors of computer room air conditioners, whether the computer room air conditioners are used to the data centre or not.